FORS 1 and FORS 2

FOcal Reducer and low dispersion Spectrograph

“Of all instruments at Paranal, this one is the Swiss Army knife”. This is the way Henri Boffin, the instrument scientist behind the FOcal Reducer and low dispersion Spectrograph 2 or FORS2, describes the instrument that is most in demand at ESO's Paranal Observatory. The key to success is that FORS2, installed on UT1 (Antu) of the Very Large Telescope (VLT), is able to study many different astronomical objects in many different ways.

But FORS2 can also take spectra of one (eso9920r), two or even several tens of objects in the sky simultaneously (eso0223b). “When used as a spectrograph, FORS2 disperses the light into very sophisticated rainbows that help astronomers study chemical composition or estimate the distances of remote objects,” says Boffin.

And this is not all. FORS2 can also measure the polarisation of light and is therefore used at the VLT to determine whether some astronomical objects have strong magnetic fields.

Observations with FORS2 and its twin brother FORS1 (decommissioned in 2009) have together led to almost 1800 papers in scientific journals as of 2014, with an average of about 100 scientific papers per year. “Basically, whatever you can think of, you can do it with FORS2. Apart from making the coffee the astronomers need at night!”

This raw image, straight from the instrument, was used, together with many others, to produce the main photo at the top of this page. The images taken with astronomical instruments are always monochromatic: the information on the colours is obtained by taking exposures through different glass filters. The thick black line cutting the field is the gap between the two detectors in the camera. Long “bleeds” caused by bright stars saturating the detector are also visible.

A raw spectrum obtained with FORS

A long, narrow slit isolates a small strip of sky. On this image, the slit is vertical. The spectrograph then splits the light from the slit into its individual colours, each point of the slit forming a horizontal rainbow. In this spectrum, the light from a distant galaxy appears as a faint horizontal line, peppered with bright dots corresponding to the colours emitted by the gas in the galaxy.